The present invention relates to a speech circuit which controls sidetones and received speech sounds in a communication equipment, a telephone set and the like.
In a speech circuit of a voice communication equipment, by providing certain attenuation to an input signal from a microphone and feeding it back to a speaker of the same speech circuit (which fed-back signal is commonly referred to as a sidetone), a talker is allowed to listen to the vocal sounds uttered by himself and hence continue conversation with ease. FIG. 3 is a block diagram showing an example of a conventional sidetone control circuit. Conventionally, the sidetone is controlled by a hybrid circuit surrounded by the broken lines. Next, the flow of a signal will be described for each functional block. An input signal a to a microphone 31 is provided onto a circuit via a hybrid transformer 33. On the other hand, a sidetone signal b is attenuated by an attenuator 32 as predetermined and then input into an adder 34. The adder 34 is simultaneously supplied with a received signal c from a communication line via the hybrid transducer 33; the adder 34 adds together these two signals and outputs a signal d. The output signal d is radiated out by a speaker 35 into space.
With such a receiving sidetone control circuit, the input signal a is always attenuated as predetermined and turned into the sidetone signal b, which is added by the adder 34 to the received signal c to form the output signal d. This operation is carried out regardless of the level of background noise.
Hence, when the background noise at the talker side, which is input into the microphone 31, is high in level, a sidetone signal (noise) of a large level proportional to the background noise is superimposed on the other party's voice or sound which is the received signal c. This degrades the articulation or clarity of the received speech voice. Moreover, since the received signal c is output at a fixed level independently of the background noise level at the talker side, the received speech sound radiated out from the speaker 35 is readily masked with background noise. This leads to the degradation of the articulation or clarity of conversations. Such a serious problem as the degradation of articulation frequently arises in telephone conversations at places where the background noise level is very high, for example, at public telephone sets installed in the Japanese super express trains.